This invention relates generally to the synthesis of olefin oxides, and more particularly to an economical and safe process for synthesizing propylene oxide and other olefin oxides.
Co-pending application Ser. No. 09/886,078 filed Jun. 20, 2001 and assigned to the assignee hereof is incorporated herein by reference. The co-pending application discloses and claims a process for synthesizing alcohols and ethers from alkanes. The process involves reacting an alkane with bromine to form the corresponding alkyl bromide and hydrogen bromide. The alkyl bromide and the hydrogen bromide are reacted with a metal oxide to produce the corresponding alcohol and/or ether, and metal bromide. The metal bromide is oxidized to form the original metal oxide and bromine, both of which are recycled.
The present invention employs a similar procedure to synthesize olefin oxides, particularly propylene oxide. Propylene oxide has heretofore been produced using a wide variety of procedures, none of which is particularly satisfactory.
The oldest and most widely used procedure for preparing propylene oxide is the propylene chlorohydrin process. An early propylene chlorohydrin technique involved electrolyzation of propylene in aqueous potassium chloride to prepare propylene chlorohydrin which was then dehydrohalogenated to produce propylene oxide. At the present time propylene oxide is prepared by reacting propylene with chlorine/water to prepare propylene chlorohydrin, then reacting the propylene chlorohydrin with aqueous calcium hydroxide, sodium hydroxide or calcium carbonate to obtain propylene oxide. A major drawback to the propylene chlorohydrin process involves the fact that the manufacture of a given quantity of propylene oxide necessarily results in the manufacture of a like or greater quantity of various salts which have little commercial value. A further disadvantage of the propylene chlorohydrin process is the fact that the propylene oxide product must be separated from large quantities of water, generally through steam stripping.
Propylene oxide can also be manufactured utilizing the ethylbenzene process. As currently practiced the ethylbenzene process involves reacting ethylbenzene with oxygen to generate ethylbenzene hydroperoxide which is then reacted with propylene to obtain propylene oxide and alpha phenylethanol. The alpha phenylethanol is then converted to styrene by dehydration. The major drawbacks to the ethylbenzene process involves the production of styrene in equal quantities with the desired propylene oxide and the use of ethylbenzene hydroperoxide, which is both explosive and subject to decomposition.
Cumene can also be used to manufacture propylene oxide. The cumene is oxidized to produce cumene hydroperoxide which is then reacted with propylene to form propylene oxide and cumyl alcohol. The cumyl alcohol is reduced to cumene by reaction with hydrogen over a catalyst and is recycled. The drawbacks to the cumene process include the use of large amounts of cumene hydroperoxide which is highly explosive and the consumption of expensive hydrogen.
A fourth process for manufacturing propylene oxide is known as the tert-butane hydroperoxide process. In accordance therewith isobutane is oxidized by reaction with oxygen to obtain tertiary butane hydroperoxide, which is then reacted with propylene to form propylene oxide and tert-BuOH. The drawbacks to the process include the direct reaction of butane with oxygen, the use of dangerous tert-butane hydroperoxide, and the production of tert-BuOH as a byproduct.
Still another process for producing propylene oxide is known as the hydrogen peroxide process. In accordance therewith, propylene is reacted with hydrogen peroxide in a solvent such as methanol over a catalyst. Drawbacks to the process include the fact that the reaction rate is very slow and the fact that expensive hydrogen is necessarily consumed to form hydrogen peroxide.
A sixth method of synthesizing propylene oxide involves direct oxidation of propylene. In accordance with the procedure, propylene is reacted with oxygen over a catalyst to generate propylene oxide. As will be apparent, safety considerations dictate that the process is very carefully controlled. Other drawbacks include low conversion rates, typically below 10% and low selectivity, typically below 40%.
The present invention comprises a method of synthesizing propylene oxide and other olefin oxides which overcomes the foregoing and other difficulties that have long since characterized the prior art. In accordance with the broader aspects of the invention, an olefin bromohydrin or an alkane dibromide is reacted with a metal oxide to form olefin oxide and metal bromide. The metal bromide is converted to obtain the original metal oxide and bromine, both of which are recycled.
In the process of the present invention an olefin bromohydrin and/or an alkane dibromide (such as propylene bromohydrin and/or 1,2-dibromopropane) is reacted with a metal oxide to synthesize olefin oxide (such as propylene oxide), with the corresponding metal bromide being formed as a by product. The metal bromide is converted back to the original metal oxide and bromine, both of which are recycled. The process consumes nothing other than olefin and oxygen. There is no direct contact between oxygen and olefin, and the process does not result in large amounts of HCl or Cl2 in water as in the traditional olefin chlorohydrin process. A further benefit of the process results from the easy separation of olefin oxide from the alkane dibromide rather than the separation of the olefin oxide from aqueous alkaline waste.